New class of rhizobia-forming bacteria identified

A second class of the Proteobacteria can form rhizobia, suggesting this ability is more widespread than previously thought.

David Bruce(

The Leguminosae is recognised as the largest plant family with approximately 18,000 documented species. Their success is in no small part due to their ability to form nitrogen-fixing symbioses with rhizobial bacteria. All rhizobia so far identified belong to the α-subclass of the Proteobacteria. In June 21 Nature, Lionel Moulin and colleagues at l'Institut de Recherché pour le Dévelopment, Paris report the discovery of legume nodulation by members of the ß-subclass of the Proteobacteria.

Moulin et al. characterized rhizobia from a number of tropical legumes and found that those from the nodules of Aspalatus and Machaerium were phylogenetically distant from known rhizobia (Nature 2001, 411:948-949). Analysis of strain STM678 from the South African legume A. carnosa showed it to be most closely related to the Burkholderia bacteria (~97% identity), a genus within the ß-subclass of the Proteobacteria. A subsequent PCR screen of this strain revealed the presence...

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